1. What is the direction of the magnetic field produced by this current loop inside the loop? A] upward B] downward

2. A loop of copper wire is shown. Moving the magnet up: A] causes increasing upward B flux B] causes decreasing upward B flux C] causes decreasing downward B flux D] causes increasing downward B flux E] has no effect on the flux through the loop

3. A loop of copper wire is shown. Moving the magnet up -causes increasing upward B flux. In what direction should the B field caused by the induced current be? A] up B] down

4. A loop of copper wire is shown. Moving the magnet up -causes increasing upward B flux. The loop current should oppose the flux change. So the field from the loop current should be DOWN. What direction does the current flow, viewed from above? A] CW B] CCW

5. A loop of copper wire is shown. Moving the magnet up -causes increasing upward B flux. The loop current should oppose the flux change. So the field from the loop current should be DOWN. The induced current must flow CW, seen from above, by the RHR. If we imagine the current loop as a little magnet, is the N pole A] up or B] down?

6. A loop of copper wire is shown. Moving the magnet up -causes increasing upward B flux. The loop current should oppose the flux change. So the field from the loop current should be DOWN. The induced current must flow CW, seen from above, by the RHR. N pole is down. (Field lines come out from the N pole.) The force between the magnet and the loop is: A] attractive B] repulsive DEMO!

7. A copper wire loop is shown. Moving the loop up: A] causes increasing upward B flux B] causes decreasing upward B flux C] causes decreasing downward B flux D] causes increasing downward B flux E] has no effect on the flux through the loop

8. A copper wire loop is shown. Moving the loop up causes decreasing downward flux. To oppose this change (i.e. this decrease) the field within the loop from the induced current must point: A] up B] down

9. A copper wire loop is shown. Moving the loop up causes decreasing downward flux. To oppose this change (i.e. this decrease) the field within the loop from the induced current must point down. Thus, the induced current in the loop must flow (seen from above): A] CW B] CCW

10. A copper wire loop is shown. Moving the loop up causes decreasing downward flux. To oppose this change (i.e. this decrease) the field within the loop from the induced current must point down. Thus, the induced current in the loop must flow (seen from above) Clockwise, by RHR.

11. More about motional EMF A square loop is pulled through a constant B field. What is the magnitude of the motional emf? A] 0 B] vBL C] 2vBL D] vBL 2

12. More about motional EMF Although there is magnetic flux through the loop, the amount is NOT changing with time. So emf = 0. If the electric potential at b=0, what is the electric potential at point a? A] 0 B] vBL C] 2vBL D] vBL 2

13. The magnetic field DOES act on mobile charges in the front and trailing sides, pushing + charges up (or - charges down). This continues for a few ns, until there is a sufficient excess of + charges on side a to create an electric field that opposes further charge motion. Answer B. V=EL=vBL. This is just the “Hall Effect” ! Although there is a potential difference between a and b, there is no emf around the loop… An emf around a loop means “you can go downhill all the way around, back to where you start!”

14. Here, the left side is not moving, so there is no magnetic force on the initially stationary charges on that side. There is magnetic force on the charges on the right side, pushing positive charges up. Each charge acquires an energy = qvBL = force x distance. That energy is then lost as the charge “slides downhill”, through the lightbulb, heating it. Where does the energy come from that lights the bulb? It cannot come from the B-field, as that is unchanging while the bar is sliding. Answer: you must use force to pull the right side at constant v. How much force? Ans: Power = Fv = I 2 R

15. More on Solenoids Long solenoids have spatially uniform B inside (from Ampere’s law) If the current is increased linearly with time, the B field will increase linearly with time. In this case, the field is out of the page (top view) and increasing with time. If this is done, what will be the direction of the induced E field at point b, distance r from the axis? On the top view: A] up B] down C] left D] right E] out of the page